Lindsey does not believe in love. More accurately, she does not believe in love happening to her. She can feel it way deep in her gut. And when her gut feels strongly about something it is rarely wrong. Rarely. She just can't believe that someone could actually fall for her. She thinks that she's too simple and weird for someone to be romantically inclined with her. Nonetheless, she is a hopeless romantic.
Levi does not believe in love. And he does not want to believe in it --at least he thinks so. Like that one movie said, "love is magical comfort food for the weak and uneducated". He believes that love is just a myth that people have created to make life a little bit more bearable to live in.
Luis, however used to believe in love. Now he doesn't, hence the term "used" . His fate in this so called "love" has long ago dissipated along with the disintegration of his parents marriage. And ever since then he refused to believe in it. He thinks it is absolutely pointless to do so.